Glycerol esters of long-chain fatty acids are conventionally used as coating agents for expandable, finely divided styrene polymers. The glycerol esters are mixed in powder form by pug milling with the polymers (DE-B No. 23 60 901) or used as a solution, or an aqueous dispersion or oganosol (cf., inter alia, DE No. 29 32 321, DE No. 33 17 69 and the state of the art cited therein). In many cases, the glycerol esters have also been employed together with other coating media, such as calcium stearate, magnesium stearate, or zinc stearate, or bis-stearylethylenediamine.
The commercially available glycerol esters (partial glycerides) used as coating agents are mixtures of mono-, di-, and triglycerol esters with a high proportion of monoesters (monoglycerides). The 1-monoester content is usually between 35 and 45% by weight, but can also be up to 90%. The proportions of triesters (triglycerides) in the commercial products varies; this proportion is about 9% and generally does not exceed 15% by weight. Higher triglyceride contents, however, can be conveniently provided. The pure triglycerides are inexpensive commercial products.
The commercially available glycerol esters (partial glycerides) used as coating media preferably contain stearic acid as the fatty acid and, depending on type of manufacture and composition, 0-6% by weight of free glycerol. They are divided into self-emulsifiable (se) and non-self-emulsifiable types (nse). The self-emulsifiable, soap-containing glycerol esters contain potassium soap or sodium soap and normally a higher proportion of free glycerol (up to 6%) than the non-self-emulsifiable types. These soap-containing glycerol esters are used, inter alia, for the production of aqueous dispersions; they have the drawback that the viscosity of the dispersions produced therefrom fluctuates widely among batches shipped. Therefore, the agitating installations, for example, must be designed with a considerable safety margin in the manufacture of the aqueous dispersions. This makes handling more difficult and impairs economy; thus, concentrations of 20% by weight, for example, can usually not be exceeded, due to viscosity considerations.
These difficult conditions also apply to dispersions containing, besides the self-emulsifiable glycerol esters, the metallic soaps zinc stearate or magnesium stearate. Such aqueous dispersions are advantageously used for coating the surface of expandable, finely divided styrene polymers since uniform coatings can be attained without significant problems. In particular, in those cases where metallic soaps are also to be applied in addition to glycerol esters, the use of dispersions likewise containing the metallic soap ensures that all finely divided styrene polymers receive the metallic soap and the glycerol ester in the desired ratio.
The non self-emulsifiable glycerol esters are soap-free. Their content of free glycerol is below 2% especially below 1.5% by weight. They cannot be used in their existing form for the production of aqueous dispersions.